Transparency needed for Ai T-shirt logo - How do you do this for cmyk print?

Question

I have a logo created in Ai CS4 and it need to have a transparnt background (no white) because it will be printed on t-shirts, some not white, and a business card, but on other than white paper. What file format should I save this in for there to be both Transparency and in CMYK format for print?
Is there a rule to remember when I want transparency AND I am going to print in CMYK?
PNG seems to give the trans, but it says it is RGB color. Is this what I would have to send to a printer? Is this correct?
In PS, it was fine to save with transp, but what is the best method, I am so very confused - Any experts out there?

Answer 1

There is no need for raster "transparency" in order to "eliminate the white background" in a project desitined for CMYK printing. There is no white ink involved. There is no process color channel in a CMYK raster image.
In other words, white in the Illustrator document (and white pixels in a CMYK raster image) do not mean "white"; they mean "no ink." So obviously, where no ink is printed, the color of the substrate (be it paper or textile) "shows through".
For textile printing of process color, however, it is often necessary to create a separation plate for white ink, because CMYK process assumes translucent inks, which do not show up well on dark substrates. In such a situation, it becomes necessary to create a separate image that will be actually be printed with an opaque white ink; the CMYK process colors are then printed on top of that. But doing that involves deliberately setting up a spot color in Illustrator, and/or a separate channel in the raster image(s). A normal CMYK or RGB image, when color-separated, does not produce a spot white plate just because there are white pixels in the image.
It's common-sense if you think about in physical terms. You have an image consisting mostly of blue and green. If you were to actually paint this on, say, a pastel yellow shirt, and did not want a white background, what would you do? You'd paint with some blue paint. You'd paint with some green paint.
Or, think of it this way: Print the image to a white piece of paper in your desktop inkjet printer. What does the printer do? It prints little dots of Cyan, Magenta, Yellow, and Black inks. Does it print little dots of white ink if you instead insert a piece of colored paper? Of course not. The printer has no idea what color the paper is, and even if it did, it couldn't automagically generate white ink.
Your post suggests you do not want a white background or underlay. If I were preparing that image for screen printing on textiles, I would not print it as CMYK process. I would make it a 2-channel image, and print it with an opaque blue and an opaque green ink. That would work for all but dark substrates. If you are printing this onto pastel substrates, it will likely work fine even if printed using CMYK process inks.
JET